To enable a person to have safe access to the part of the cell which is separated from the line conductors by the opening of the switch, the latter is generally either combined with an isolator, or constituted in such a way that one of its elements, such as a bushing, effecting a separation, is connected with a metallic curtain which comes between the contacts of the device in the open position. These solutions can be sufficient when the extinguishing of the arc is effected in air. But if the cut-out takes place in a fluid having a dielectric rigidity greater than that of air, for example a gas such as sulphur hexafluoride, the preceding arrangements are unsuitable. They generally require the rotating of a bushing whose moment of inertia is fairly high and they thus lead either to a bulky solution if the isolator is in series with the switch, or to a solution which absorbs a great amount of energy, which is hence incompatible with the use of simple automatic equipment, if the bushing is combined with the mobile part of the switch whose movement must be fairly rapid.